This invention relates to an improved solar collector system and more particularly to an improved supporting and drive axle arrangement for use with solar collecting panels. In another aspect, this invention relates to apparatus for minimizing energy losses and consumption in movable solar collecting systems and more particularly to improved systems having reduced frictional losses and reduced thermal losses from the thermal receiving tube.
Recent economic and political developments have resulted in a drastic increase in the cost of conventional energy sources such as crude oil, natural gas and the like. Due to the increased prices of conventional energy sources and because of the very real and potential shortages of conventional energy sources such as crude oil, natural gas and the like, many alternative sources of energy are being investigated. One particularly promising alternative source of energy is the recovery and use of solar radiation or "solar energy". While it has long been known that solar energy is a vast untapped source of energy, conventional energy sources such as crude oil, natural gas and the like, have been so cheap that solar energy recovery could not be justified from an economic standpoint. Now, because of the changing economic conditions affecting conventional energy and because of possible shortages of such conventional energy, solar energy represents a viable source of energy.
Many types of solar collector systems have been investigated. Such solar collector systems include tubes, mats and other large surface area solar collectors that contain some type of working fluid that is heated by merely placing such large solar collectors in the sun. Such solar collectors are effective in heating the working fluids only a few degrees above ambient temperature. Additionally, such large solar collectors are relatively expensive. Thus, the use of such large collectors is unattractive because a large capital investment must be made to recover only low grade energy.
Recently, a considerable amount of interest has been directed toward improved types of solar collectors to gather solar energy and concentrate the solar energy into a relatively small area to thereby achieve high temperatures. Such solar collectors usually involve some type of lens or prism reflective means or some type of reflective surface means that will concentrate solar radiation from a relatively large area onto a relatively small target or collector means. The use of highly reflective surfaces or mirror-type surfaces are very effective for gathering solar radiation striking a relatively large effective area and focusing or concentrating the radiation onto a relatively small target area or energy receiver. Such types of solar collectors include a collection of many individual flat mirrors that can be focused onto one small target or energy collector, as well as various types of curved and shaped reflective surfaces that will focus the thus collected solar energy onto a relatively small surface or energy receiver.
One particularly preferred type of solar collector is a parabolic dish reflector which will gather solar radiation and reflect the radiation onto a small target or energy receiver which is located at the focal point of the parabola. Because of the relatively high cost of constructing a true parabolic surface, parabolic dish reflectors are not widely used for low cost energy recovery from the sun.
Recently, parabolic trough reflectors have been investigated as relatively low cost types of solar collectors. The parabolic trough reflectors have proven to be much less expensive to manufacture than parabolic dish reflectors. Some of the most effective parabolic trough reflectors utilize a relatively large reflector surface that is formed by constructing an elongated trough-like means with the walls of the trough having a constant parabolic shape whereby the focal point of the parabolic trough lies along a relatively straight line above the trough. Thus, the concave parabolic trough solar collector can be equipped with a target or energy receiver that is disposed along the line formed by the focal point of the parabolic reflector. By using such a reflector means, solar radiation which strikes the concave surface of the solar collector will be reflected and concentrated onto the focal point of the parabola and can be captured by an energy receiver located at or near the focal point of the parabolic surface.
In order to maximize the amount of solar energy that can be captured using reflector type solar collectors, it has been found highly desirable to utilize a movable solar collector. By using the movable solar collector, as opposed to stationary collectors, the efficiency of the solar collector system can be greatly increased. Thus, the use of movable solar collectors that can be effectively aimed at the sun will greatly increase the overall energy recovery efficiency of the system. In the case of parabolic shaped solar collectors, maximum energy recovery is obtained when the axis of the parabola is aimed directly toward the sun.
Conventional methods and apparatus for supporting and aiming solar collectors toward the sun are rather crude and simple. For example, support means for solar collectors usually include axles or support rods on which the collector means is supported with the axles being journaled into bearing means or bushings whereby the supported reflector can be rotated to presumably point the reflector surface directly toward the sun. This type of apparatus for supporting the solar collector is wrought with many problems. Specifically, it is extremely difficult to install such a type of system in such a manner that the bearings and axle means are in precise alignment. This is extremely critical, especially when the solar collector is quite large and the bearing means are a considerable distance apart. It is, of course, not uncommon to have solar collectors that are up to at least twenty feet in length with the collector surface being supported only at the outer ends by journaling the axle means into bearing means that are carried by pylons that are at least twenty feet apart. Even if the system can be installed with the bearing means being precisely aligned to receive the axle means, problems still exist due to an uneven shifting or settling of the support pylon means. It is, of course, well known that many solar collectors are installed on roof tops. The weight of the solar collectors, as well as varying loads from wind, rain, ice, snow and the like, on the roof structure, will cause support pylons for solar collectors to move and shift to a considerable degree. Such a movement and shifting of the pylons will, of course, cause a misalignment of the bearing means. Still another problem connected with conventional methods and apparatus for supporting solar collectors is due to the fact that varying wind loads on the solar collector structure itself, will often cause bending and twisting forces to be exerted on the solar collector and its supporting axles to cause misalignment of the axles with the bearing means.
Another source of misalignment is the thermal expansion of the solar collectors themselves. The collectors are often made of aluminum which has a relatively relatively high coefficient of thermal expansion. The panels are exposed to daily temperature cycles on the order of 20.degree. F. to 40.degree. F. and annular variations of at least 100.degree. F. The supporting materials, for example the earth, typically has a lower expansion coefficient and is somewhat insulated from ambient thermal cycles. As a result the length of a single 20 foot panel may vary by 0.1 inch or more on a daily basis, and the overall length of a six panel array may vary by 0.6 inch or more on a daily basis. Annual variations may easily be five times as much as the daily variations. These changes in panel length must be taken up in strain in the bearing assemblies or in actual movement of the pylons, either of which can cause misalignment and binding of the bearings.
In all cases where there is even the slightest amount of misalignment between the bearing means and the axles that carry solar collectors, rotation of the solar collectors will become extremely difficult, if not impossible. Thus, even if the system is designed to withstand misalignment of the axles and bearing means, a considerable amount of force and energy will be necessary to cause any desired rotation of the solar collector.
Conventional means for actually causing the rotation of solar collectors to allow them to be aimed at the sun to maximize energy collection utilizes a mechanical drive system. The mechanical drive system will normally be a system of gears that are driven by some type of motor means. The mechanical gears can include a conventional gear train with interlocking cog gears, or a worm gear arrangement can be utilized. Thus, by affixing at least one of the gears to either the axle of the solar collector or to the solar collector structure, itself, and having such gear means, motor means or the like, the solar collector can be selectively aimed at different points in the sky by activating the crank, motor or the like. Unfortunately, such mechanical devices for moving a solar collector simply do not stand up well over long periods of use because the gear mechanism invariably wears and develops "back lash". The wearing of the gear surfaces and the development of back lash will cause an erratic movement of the solar collector as the collector is being pointed toward the sun. This is especially noticeable when the solar collector is exposed to buffeting wind loads. Another problem connected with the use of mechanical drive means occurs in the amount of power that is necessary to drive such mechanical devices. It is often necessary to utilize heavy and expensive variable speed motors. While attempts may have been made to utilize constant speed motors and to periodically activate the motors in response to a command signal, it has been found that the power consumption for the intermittent activation of such motors is quite high. There are several instances where the amount of power required to move the solar collector to track the sun as it traverses the sky often approaches the amount of energy that can be recovered by using the solar collector.
It is thus very apparent that there is a need for an improved support system and system for moving solar collectors. It is also evident that there is a tremendous need for improved solar collector systems that can be installed in locations where the support structure may move and shift without decreasing the efficiency of the system. There is also a need for a support system which allows for some axial movement of solar collectors relative to support axles to eliminate stresses from thermal expansion. It is also evident there is a need for an improved solar collector system whereby the solar collector can be moved in response to a command with very little power usage and without the problems connected with conventional apparatus for moving solar collectors.